1. Field of the Invention
The present invention relates to a light beam emitting apparatus including a plurality of light emitting points, a light scanning apparatus including the light beam emitting apparatus, and an image forming apparatus including the light scanning apparatus.
2. Description of the Related Art
Light scanning apparatus are used in image forming apparatus such as an electrophotographic copying machine and a laser beam printer, which are configured to form images by using an electrophotographic process. In the electrophotographic process, the light scanning apparatus generally scans a photosensitive member with a light beam (laser beam) which blinks on and off in accordance with image signals so as to form an electrostatic latent image on the photosensitive member. The electrostatic latent image is developed with developer (toner) into a developer image. The developer image is transferred onto a recording medium. The developer image on the recording medium is fixed onto the recording medium by heat and pressure. In this way, images are formed.
The light scanning apparatus includes semiconductor lasers as light sources. The semiconductor lasers are each configured to emit a light beam which is modulated in accordance with image signals. The light beam is converted into a substantially parallel light beam by a collimator lens. The substantially parallel light beam is deflected by a rotary polygon mirror of a deflecting device. The deflected light beam is imaged on the photosensitive member by imaging optical elements as a light spot which moves in a main scanning direction on the photosensitive member. The light beam repeatedly scans on the rotating photosensitive member in the main scanning direction so that the electrostatic latent image is formed on the photosensitive member.
In the following description, the main scanning direction refers to a direction (a direction in which the light beam is deflected (reflected) by the deflecting device) perpendicular to a rotation axis (or pivot axis) of the deflecting device and an optical axis of imaging optics. A direction of an optical axis of incident optics between the light source and the deflecting device is different from a direction of an optical axis of scanning optics between the deflecting device and the photosensitive member. A sub-scanning direction refers to a direction (a direction which is parallel to the rotation axis (or pivot axis) of the deflecting device) perpendicular to the optical axis of the imaging optics and the main scanning direction. A main scanning cross-section refers to a plane including the optical axis of the imaging optics and the main scanning direction. A sub-scanning cross-section refers to a cross-section which includes the optical axis of the imaging optics and is perpendicular to the main scanning cross-section.
In recent years, in order to increase a processing speed and a scanning density of the light scanning apparatus, multi-beam light sources have been employed. A surface emitting laser (a vertical cavity surface emitting laser: VCSEL) can be easily used in arrays, and hence there have been proposed a wide variety of light scanning apparatus using a surface emitting laser array as a light source of the light scanning apparatus.
In the light scanning apparatus, a direction of light beams to be emitted from the light source (optical axis direction) influences optical characteristics of imaging optics. Thus, the optical axis direction needs to be adjusted with accuracy of the scale of micrometer. When an emission position of the light source is displaced back and forth with respect to a predetermined position along the optical axis direction (direction of depth of focus), an error caused by the displacement is expanded about tens of times to hundreds of times on the photosensitive member, resulting in an error in a focal position of the light beam on the surface of the photosensitive member (hereinafter referred to as “error of focal position”). In other words, the error of focal position expanded in accordance with an axial magnification in the imaging optics occurs.
Specifically, for example, when a spot diameter of the light beam on the photosensitive member is set to 50 μm, a depth of focus is set to 4 mm, and field curvature is set to 2 mm, tolerance of error of focal position in the direction of depth of focus is 2 mm. In this case, when the axial magnification in the light scanning apparatus is set to a magnification of one hundred times, tolerance of error of the emission position of the light source in the optical axis direction is 20 μm or less.
When the relationship described above is applied to a light beam emitting apparatus including a dual-beam semiconductor laser array in which an interval between two light emitting points is 50 μm, an inclination of a light source corresponding to a difference in position along the direction of depth of focus between the two light emitting points is permissible up to 21.8°. Meanwhile, in a light beam emitting apparatus including a multi-beam semiconductor laser array having several light emitting points to several tens of light emitting points, tolerance of the inclination of the light source is small. For example, in a case where an interval between two light emitting points at both ends out of ten light emitting points is 200 μm, in order to suppress the error along a direction of depth of focus between the two light emitting points to 20 μm or less, the inclination of the light source needs to be set to 5.7° or less. In particular, the surface emitting laser array include a large number of light emitting points, and hence need to be mounted with higher accuracy.
In view of the above-mentioned circumstances, Japanese Patent Application Laid-Open No. 2004-006592 proposes a method of mounting a reference surface of a light source including a surface emitting laser array perpendicularly to an optical axis of a light beam emitting apparatus with high accuracy. In Japanese Patent Application Laid-Open No. 2004-006592, a reference surface parallel to a plane including light emitting points of the surface emitting laser array is provided on an upper surface of a package portion of the light source. The reference surface of the light source is brought into abutment with a reference surface (three abutment portions) of a holding member configured to hold the light source. In this way, the reference surface of the light source is mounted perpendicularly to the optical axis of the light beam emitting apparatus with high accuracy.
However, as disclosed in Japanese Patent Application Laid-Open No. 2004-006592, in order to hold the light source by the holding member irrespective of array directions of the plurality of light emitting points, heights of the three abutment portions in the optical axis direction, which serve as the reference surface of the holding member, need to be set with significantly high accuracy.
In other words, there is a problem in that, in order that an array line (first straight line) connecting two light emitting points which are farthest from each other be arranged perpendicularly to the optical axis of the light beam emitting apparatus, differences in height in the optical axis direction among the three abutment portions need to be significantly reduced.